A stimulus sensitive composition generates an acid or a radical on receipt of an external stimulus, such as active light rays, radiations or heat, and undergoes chemical reaction by the action of the generated acid or radical thereby changing its physical properties in the stimulated area. Preferably, it is a pattern forming material that changes its solubility in a developer between an irradiated or heated area and a non-irradiated or non-heated area to provide a pattern on a substrate.
A variety of acid generators capable of generating an acid when irradiated with active light rays or radiations have been proposed for use in such stimulus sensitive compositions. For example, sulfonium salts having an oxoalkyl group or a cyclic oxoalkyl group are disclosed in JP-A-2002-116546, JP-A-2001-187780, EP 1113334A, JP-A-10-133371, and JP-A-10-73919. The following compounds are among them.

However, these known acid generating compounds are unstable to heat, liable to deterioration with time, or excessively hydrophilic. Pattern formation using resist compositions containing them results in tapered profiles or rounded top profiles.